fix(nunit): properly report ignored test cases

[delatrie]

Context

Suppose we have the following test fixture:

using Allure.NUnit;
using Allure.NUnit.Attributes;
using NUnit.Framework;

[AllureNUnit, AllureDisplayIgnored]
public class IgnoredTestCases
{
    [TestCase("foo")]
    [TestCase("bar")]
    [Ignore("baz")]
    public void IgnoredTestMethod(string data)
    {
    }
}

When we run the tests and generate the report, the report will contain a single parameterless test result that corresponds to the test method (not a specific test case).

The PR fixes this issue by properly collecting ignored leaves from the test hierarchy provided by NUnit.

Screenshots

Before:

After: